1. Field of the Invention
The present invention relates to an imaging element for electronic endoscopes and an electronic endoscope equipped with the imaging element.
2. Description of the Related Art
In the field of medicine, electronic endoscope systems are used as diagnostic systems for examining internal sites of the human body.
In general, such an electronic endoscope system includes a light source device and an endoscope which is removably mounted (connected) to the light source device. The end scope is provided with a CCD imaging sensor (imaging element) and an imaging optical system which are provided at the tip end portion thereof.
As the imaging element, an imaging element is equipped with a CMOS (Complementary Metal Oxide Semiconductor) sensor (hereinafter, referred to as “CMOS type imaging element”) attracts attention.
This CMOS type imaging element has various advantages. For example, in the case of the CMOS type imaging element, a CMOS sensor and its circumference circuits can be formed on the single substrate (on the same chip) according to the general MOS manufacturing process.
FIG. 10 is a plane view showing the imaging element used in the conventional electronic endoscopes.
The imaging element 100 shown in this figure is a CMOS type imaging element, which comprises a CMOS sensor 120, a control circuit 111 which performs timing control, and a signal processing circuit 112 which performs signal processing. The control circuit 111 is arranged at the upper side of the CMOS sensor 120 in FIG. 10, and the signal processing circuit 112 is arranged at the right side of the CMOS sensor 120 in FIG. 10.
The CMOS sensor 120 includes an imaging region 122. The imaging region 122 is composed of an effective imaging region 123 and a shading region 124 which is provided around the periphery of the effective imaging region 123. This shading region 124 (shown by the slanting lines in FIG. 10) is provided for detecting are ference level of optical black. The shading region 124 is normally referred to as an “optical black (optical black portion).”
In such an imaging element 100, as shown in FIG. 10, the center 150 of the effective region 123 of the imaging region 122 is not aligned with (coincided with) the center 140 of the imaging element 100 on a light-receiving surface 121.
The imaging element 100 and the imaging optical system are provided inside a predetermined opening portion in the tip portion of the endoscope. However, as described above, the center 150 of the effective region 123 is displaced from the center 140 of the imaging element 100. Therefore, if the imaging element 100 is mounted in the opening portion without any adjustment, the center of the imaging optical system (i.e., the intersection point between the optical axis of the imaging optical system and the light-receiving surface 121) is not aligned with the center 150 of the effective region 123.
For this reason, when the imaging element 100 is inserted into such an opening portion during assembly of the electronic endoscope, a centering adjustment (that is, an adjustment for correcting eccentricity) is carried out.
This centering adjustment is carried out by providing an adjustment spacer (centering means, not shown) outside the imaging element 100 using a predetermined jig so that the center 140 of the imaging element 100 containing the spacer is aligned with the center 150 of the effective region 123.
However, such centering adjustment operations are difficult to carry out automatically, and therefore a significant amount of time and labor is required for assembling the electronic endoscope.
Further, because the spacer has to be provided outside the imaging element 100, the additional space for housing such spacer is needed inside the tip portion of the electronic endoscope, which results in a problem in that the diameter of the electronic endoscope is enlarged to such an extent.